Abstract: As a low rank coal with abundant reserves and high reactivity, lignite is is very suitable for gasification. However, there exist several issues, including high internal water content, susceptibility to spontaneous combustion, and low concentration of the prepared coal water slurry, which lead to the challenges in large-scale gasification utilization. At present, there are many studies on the preparation of coal water slurry through the upgrading or modification of lignite, but there are few reports on the preparation of high-concentration coal water slurry through low-temperature pyrolysis coupled with particle size grading. In order to study the influence of low-temperature pyrolysis and particle size grading on the preparation of high-concentration coal water slurry from lignite, Yili lignite was used as the raw material and semi coke produced through pyrolysis, as the research object. The effects of low-temperature pyrolysis on the properties, surface functional groups, hydrophilicity and hydrophobicity, and Zeta potential of lignite were investigated through infrared spectroscopy, contact angle, and Zeta potential analysis. The slurryability tests were conducted using rod milling and particle size grading processes, to explore the effects of low-temperature pyrolysis and slurrying processes on coal slurry concentration, particle size distribution, and rheological properties. Research has found that the moisture and volatile content of lignite decreased through low-temperature pyrolysis, and the O/C ratio decreased from 22.29% to 6.61%. With the increase of pyrolysis temperature, the alkyl side chains on the surface of lignite are removed and the oxygen-containing functional groups are reduced; As the contact angle increases, the wettability of the surface improves. Simultaneously, the increase in ash content and pore size leads to an increase in the absolute value of the Zeta potential, which improves the stability of the slurry. Benefiting from low-temperature pyrolysis, a large number of oxygen-containing functional groups and side chain groups on the surface of lignite are removed, which reduces the ability of hydrophilic groups on the surface of lignite to convert free water into bound water. The content of carbon skeleton increases, indicating that pyrolysis raises the coal rank, as a result, the concentration of lignite slurry has been increased, and the amount of coal slurry additives used in slurry preparation has been reduced. Using lignite and pyrolysis semi coke as pulp materials, the concentration of coal water slurry prepared by single rod milling process are 54.5%, 59.4%, and 60.5%. The semi-coke produced through pyrolysis is slurried by particle size grading process, which can increase the coal slurry concentration to a maximum of 66.5%. During pyrolysis, the structural integrity of coal particles is compromised as volatiles and moisture evaporate, which enhances the grindability of lignite and reduces the grinding time for slurry preparation. The coal slurry, prepared by particle size grading process, exhibits a bimodal particle size distribution, where fine particles fill the gaps between larger ones, effectively reducing the viscosity of the coal slurry. Low temperature thermal decoupling particle size distribution can improve the particle size distribution of coal slurry, reduce the viscosity of coal slurry, optimize the fluidity of slurry, and prepare high concentration lignite coal water slurry.